Method of carbonate-bonding specular hematite and resulting product



United States Patent M METHOD OF CARBONATE-BONDING' SPECULAR HEMATITE AND RESULTING PRODUCT Roger T. Johnson, Monroeville, and Allan Sass, Penn Hills Township, Allegheny County, Pa., assignors to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Jan. 20, 1966, Ser. No. 521,826 8 Claims. (Cl. 75-6) This invention relates to an improved method of making carbonate-bonded agglomerates of specular hematite and to the resulting product.

Specular hematite is produced as a coarse-grained concentrate, typically containing more than 90 percent plus 100 mesh particles. Before the concentrate can be charged to a blast furnace, it must be formed into agglomerates large enough not to be blown from the furnace. Probably the most common method of agglomerating iron ore is to pelletize it. In a conventional pelletizing process, the ore is formed into green pellets or balls, which are fired at an elevated temperature to sinter the individual particles. Specular hematite is difiicult to pelletize. The usual practice has been to regrind the material until all the particles are of extremely fine size and then pelletize the reground particles.

It is known that iron ore fines can be agglomerated by a carbonate-bonding process in which a moist mixture of iron ore and alkaline earth metal oxide or hydroxide is formed into green agglomerates and treated with CO gas. It is also known to include coal fines in the mixture to increase the mechanical strength of the agglomerates after they are heated to a temperature at which the resulting alkaline earth metal carbonate calcines. The coal forms a coke bond which holds the agglomerates together until the ore particles sinter and form their own bond. However, carbonate bonding processes have not been successful as applied to specular hematite, either in the as-received condition or reground for pelletizing. When carbonate bonded specular hematite agglomerates are tested under simulated blast furnace conditions, they swell and become quite Weak.

An object of the present invention is to provide an improved method of carbonate-bonding specular hematite particles and obtaining agglomerates which resist swelling.

A further object is to provide a method in which we accomplish the foregoing objective by using specular hematite particles of a selective size consist in an otherwise conventional carbonate-bonding process.

A further object is to provide an improved carbonatebonded specular hematite product which resists swelling.

According to our invention, we regrind as-received specular hematite until it is 60 to 95 percent minus 325 mesh, or preferably 70 to 80 percent minus 325 mesh. These limits are critical. If more than 80 percent of the reground material is minus 325 mesh, the subsequent product is adversely affected. If more than 95 percent is minus 325 mesh, the benefits are lost altogether. This phenomenon is quite distinct from what one encounters in pelletizing the same material, where the more finely it is ground the stronger the pellets. Preferably we mix the reground material with up to 50 percent as-received specular hematite. This not only diminishes the amount of grinding, but actually furnishes better agglomerates, as the examples hereinafter demonstrate. We then form the material into carbonate-bonded agglomerates by conventional procedures. Essentially this consists in mixing the material with about to 20 percent alkaline earth metal oxide or hydroxide, up to about percent moisture, and preferably about 3 to 10 percent bituminous coal fines. The resulting mixture next is formed into green agglomerates, as with a disc or drum pelletizer or a 3,370,936 Patented Feb. 27, 1968 briquetting press, the mositure content is adjusted, and the green agglomerates are treated with CO gas.

As a specific example, we produced green agglomerates from specular hematite, 7 to 9 percent hydrated lime, and 3 to 5 percent bituminous coal fines by use of a conventional disc pelletizer. We added sufficient water to provide a moisture content of about 10% in the mix while forming the green agglomerates. We dried the green agglomerates to a moisture content of about 1 /2 to 3 percent and exposed them to CO gas. We tested the resulting carbonate-bonded agglomerates formed of various grinds and mixtures as hereinafter listed. The test consisted in heating the agglomerates. to 1900 F. in an electrically heated sample tube through which flows reducing gas similar in composition to that found in the stack of a conventional blast furnace (that is, a simulated blast furnace test). Following this test we screened the agglomerates for 15 minutes at 4 mesh in. a conventional Re-Tap device. The results were as follows:

A. All reground concentrate B. Mixtures of as-received and reground concentrate Percent Percent +4 Mesh Regrouud Grind As-Received after lrR pn 65 71% -325 mesh 35 50 71%325 mesh 50 89 60 79%-325 mesh 40 89 50 1007 -325 mesh 50 =50 From the foregoing description it is seen that our invention affords a simple effective method of producing stronger carbonate-bonded agglomerates of specular hematite. We obtained the highest content of plus 4 mesh material following the simulated blast furnace test with a reground specular hematite which contains about 71 to 79 percent minus 325 mesh particles mixed with about 35 to 50 percent as-received.

While we have described only a single embodiment of our invention, it is apparent that modifications may arise. Therefore, we do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

We claim:

1. A method of making carbonate-bonded agglomerates of specular hematite comprising regrinding relativelycoarse as-received specular hematite to produce about 60 to percent minus 325 mesh particles, moistening the particles and mixing them with about 5 to 20 percent of a compound selected from the group consisting of alkaline earth metal oxide and hydroxide, forming the mixture into green agglomerates, and exposing the green agglomerates to CO gas.

2. A method as defined in claim 1 in which the reground particles are about 70 to 80 percent minus 325 mesh.

3. A method as defined in claim 1 in which bituminous coal fines are included in the mixture in an amount to provide a coal content of about 3 to 10 percent.

4. A method as defined in claim 1 in which asreceived specular hematite is mixed with the reground specular hematite in an amount up to about 50 percent of the specular hematite in the mixture.

5. A carbonate-bonded specular hematite product consisting of reground as-received specular hematite in which there is 60 to 95 percent minus 325 mesh material,

i t V V and a carbonate ofjan-alkaline earth metal forming a 8. A product as defined in ciaim 5 containing in eddibond for the hematite particles. 7 tion as-received specular hematite up to about 50 6. A product as defined in claim 5 in which the re- P ground particles are 70 to 80 percent minus 325 mesh. NO references cited r 7. A product as defined in claim 5 containing in addi- 0 tion 3 to 10 percent bituminous coal fines. BENJAMIN HENKIN, PFimaI'YEMmiHEII. 

1. A METHOD OF MAKING CARBONATE-BONDED AGGLOMERATES OF SPECULAR HEMATIC COMPRISING REGRINDING RELATIVELYCOARSE "AS-RECEIVED" SPECULAR HEMATITE TO PRODUCE ABOUT 60 TO 95 PERCENT MINUS 325 MESH PARTICLES, MOISTENING THE PARTICLES AND MIXING THEM WITH ABOUT 5 TO 20 PERCENT OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF ALKALINE EARTH METAL OXIDE AND HYDROXIDE, FORMING THE MIXTURE INTO GREE AGGLOMERATES, AND EXPOSING THE GREEN AGGLOMERATES TO CO2 GAS. 